One minor criteria for the diagnosis of systemic mastocytosis (SM) is a serum tryptase >20 ng/mL, which generally reflects mast cell expansion and is a useful marker of mast cell activation by established criteria. Other mediators where no such criteria have been proposed include heparin, histamine, and prostaglandin D2. Some authors have also suggested chromogranin A (CgA) should be among these markers but based on limited data. CgA itself is a 439-residue granin family protein (48-60 kD) found in the secretory vesicles of neuroendocrine tissues and is a biomarker for assessment of neuroendocrine tumors. Proton pump inhibitor (PPI) use is associated with an increase in CgA levels, as acid suppression by PPIs promotes hypergastrinemia which leads to increased CgA via gastrin-regulated enterochromaffin-like cells. We thus prospectively determined serum CgA, gastrin from FY 2017 into FY 2018 in 20 adults and 17 pediatric patients diagnosed with mastocytosis. We found that both adult and pediatric patients with mastocytosis not taking PPIs have serum CgA levels within the normal reference range and that the serum levels of CgA are significantly influenced by the use of PPIs. Biopsies stained for CgA associated with mast cells were negative. These results demonstrate that mast cells are not a significant systemic source of serum CgA. Therefore, we recommended that serum CgA not be used as a biomarker of mast cell disease, which if widely applied as a recommendation, should result in significant cost savings and help avoid confusion in the use of serum CgA determinations. Human mast cell lines have been widely used to study the pathophysiology of allergic diseases and systemic mast cell disorders. In addition, they are increasingly used to validate new therapeutic targets for allergic diseases and mastocytosis; and to screen and validate the effects of new targeted drugs. Our contribution has been the derivation and wide distribution of the LAD2 human cell line which is unique in that it has no mutations in KIT, requires stem cell factor for growth, and degranulates to IgE mediated stimuli. In FY 2018, we were able to transfect these LAD2 with KIT bearing the D816V mutation. This provides the opportunity to directly examine the consequences of this form of KIT on the biology of the human mast cell. Extracellular vesicles (EVs) associate with hematologic disorders and are reported as vectors of molecular information that effect other cell types. In FY 2018 we thus examined blood samples from patients with systemic mastocytosis (SM) and found EVs with a mast cell signature including the presence of tryptase, FcepsilonRI, MRGPRX2 and KIT. The concentration of these EVs correlated with parameters of disease including levels of serum tryptase and hepatosplenomegaly. We then questioned if SM-EVs might affect hepatic stellate cells, given the abnormal liver pathology associated with mastocytosis. We found that mastocytosis EVs are taken up by hepatic stellate cells (HSCs) and this interaction altered the proliferation, cytokine production and differentiation of these cells. This data is consistent with the conclusion that SM-EVs have the potential to influence cells outside the hematological compartment, and that therapeutic approaches for treatment of SM should in part target inhibition of the effects of EVs on target tissues. These findings are in review. Use of allele specific qPCR to identify KIT D816V in the peripheral blood of adults with mastocytosis has been shown to have value in the diagnosis and management of this disease. To examine the value of this assay in children with cutaneous mastocytosis, in FY 2018 we assessed data on 65 children with pediatric-onset cutaneous and systemic mastocytosis. Within this cohort we correlated KIT mutation status with clinical findings, serum tryptase levels and bone marrow histopathology. We found that KIT D816V in peripheral blood was exclusively found in children known to have both cutaneous and systemic disease (sensitivity of 85.2%). These findings will assist in the decision for when to perform a bone marrow biopsy in children presenting with cutaneous mastocytosis. These findings are in review.